1. Field of the Disclosure
The disclosure relates generally to processes for separating solids from a solid-liquid slurry, for example in melt-crystallization separation and purification processes. More specifically, the disclosure relates to separation or purification processes that, under certain operating conditions, advantageously prevent ingress of molecular oxygen into the processes. The processes are, therefore, more efficient than, and at least as safe as, conventional processes for separating solids from a solid-liquid slurry.
2. Brief Description of Related Technology
Solid-liquid separation methods are important in a variety of industries, including, but not limited to, the chemical industry, the pharmaceutical industry, and the water and waste treatment industry. Such solid-liquid separation methods vary, and may include, but are not limited to, vacuum or pressure filtration, centrifugation, sedimentation and clarification. In many chemical processes, these solid-liquid separation methods often play a critical role in the manufacture of particular chemical intermediates. For example, para-xylene (p-xylene or pX) is a chemical intermediate that, when purified, is useful for making terephthalic acid. Purification of para-xylene through crystallization has historically required centrifugation to achieve para-xylene purity levels of about 99.7%.
A para-xylene purification process is typically part of a much larger process of manufacturing para-xylene from a hydrocarbon feed containing mixed C8 aromatic hydrocarbons. In that larger process, a hydrated feed is typically dehydrated in a distillation tower. Any oxygen present in the distillation feed typically exits the tower with the overhead water vapor. A dry feed, of course, need not undergo this dehydration step. Following feed dehydration (if any), the dry C8 aromatic hydrocarbons are purified in a process employing various unit operations. The para-xylene lean stream produced by that purification process contains ortho-xylene (o-xylene or oX), meta-xylene (m-xylene or mX), ethylbenzene, and other components. That para-xylene lean stream typically is vaporized and reacted in the presence of a catalyst and hydrogen (in an isomerization reactor) to obtain an equilibrated mixture of xylene isomers (i.e., oX:mX:pX in a weight ratio of 1:2:1), which is then sent to a fractionation section where the C8 aromatic hydrocarbons are separated and sent to the purification process to obtain purified para-xylene.
The introduction of oxygen into the process has been found to deleteriously affect the isomerization part of the process. For example, the presence of oxygen in the feed to the isomerization reactor leads to fouling in the reactor as well as in the upstream heat exchangers. This results in reduced cycle length. At the end of the cycle, the process must be shut down to regenerate the catalyst and de-coke the heat exchangers. The presence of oxygen may be inconsequential in other processes that might use solids-liquids separators. In the process for manufacturing para-xylene from a mixed C8 aromatic hydrocarbon feed, however, it has been found that the absence or minimization of oxygen is important for efficient operation of the process.
The purification of para-xylene typically begins with a predominantly C8 aromatic hydrocarbon feed that typically includes a mixture of ortho-, meta-, and para-xylene isomers, ethylbenzene, some non-aromatic hydrocarbons, and some C9+ aromatic hydrocarbons. Typical mixtures of C8 aromatic hydrocarbons generally contain about 22 wt. % para-xylene, about 21 wt. % ortho-xylene, about 47 wt. % meta-xylene, and about 10 wt. % of other constituents (mostly ethylbenzene). Processes to separate these xylene isomers include low temperature crystallization, fractional distillation, and adsorption.
While the common separation techniques of distillation (based on the differential boiling points of mixture components) and adsorption (based on different affinities of mixture components to a solid adsorbent) are often suitable for generic liquid-liquid mixtures, crystallization requires no adsorbent, is more tolerant of various feedstock compositions, and typically requires no costly feedstock pre-treatment. In separating para-xylene from a C8 aromatic hydrocarbon feed, for example, crystallization is often preferred over adsorption and distillation because crystallization does not require a costly adsorbent (as in adsorption processes), and because xylene isomers and ethylbenzene have undesirably similar boiling points (making distillation difficult), but dramatically different melting points. Pure para-xylene freezes at 56° F. (13° C.), pure meta-xylene freezes at −54° F. (−48° C.), pure ortho-xylene freezes at −13° F. (−25° C.), and pure ethylbenzene freezes at −139° F. (−95° C.). Because para-xylene is present in these mixed feed streams in low concentrations, very low temperatures are generally required to effectively recover the para-xylene from these feed streams by crystallization.
As in any chemical process, capital and operating costs will drive decisions on which specific unit operations to employ to obtain satisfactory products and by-products. These decisions, of course, can be complicated and limited by the physical properties of the products and by-products. Moreover, these decisions can be complicated when, for example, the process needs to be designed and operated to avoid introduction (or manufacture) of contaminants. Important considerations for the recovery and purification of para-xylene via crystallization include, for example, operating costs associated with obtaining low temperature refrigeration, and capital costs associated with solid-liquid separation units. Furthermore, and as explained in more detail herein, oxygen is considered a contaminant in para-xylene manufacturing processes inasmuch as oxygen imparts inefficiencies to the processes. Oxygen may also be considered a contaminant in other manufacturing processes employing solid-liquid separation and purification processes.
Consequently, efficient solid-liquid separation and purification processes that also remove oxygen from the unit operations responsible for the separation and purification are desired. Furthermore, efficient solid-liquid separation and purification processes that also minimize or entirely avoid introduction of oxygen into the unit operations responsible for the separation and purification are desired. In the particular context of para-xylene manufacture, efficient processes for recovering and purifying para-xylene that also remove oxygen from the unit operations responsible for the recovery and purification of para-xylene are desired. Furthermore, efficient processes for recovering and purifying para-xylene that also minimize or entirely avoid introduction of oxygen into the unit operations responsible for the recovery and purification of para-xylene are desired.